Can Laser Propulsion Stabilize Satellites?

Laser-propelled spacecraft have been around in science-fiction comic books for years, but now an independent scientist has seriously proposed laser propulsion as a technique to stabilize rock-solid formations of spacecraft, thereby providing high-resolution, remote-sensing platforms. In an initial laboratory demonstration, Young K. Bae of Bae Institute in Tustin, Calif., recently demonstrated a laser-generated thrust of some 35 μN, a force he says is already adequate for many space missions.

Figure 1. Several NASA projects, such as the Terrestrial Planet Finder pictured here, envision interferometers with sensors on individual satellites. A challenge in constructing such interferometers is maintaining submicron, or even nanometer, stability within the formation of satellites. Courtesy of NASA/JPL-Caltech.Let’s back up a minute and examine why people are interested in rock-solid formations of satellites. Optics or antennas with very large apertures are required for high-resolution imaging and for collecting signals from very weak or distant sources. But large apertures can be synthesized interferometrically with a few sensors placed around the perimeter of the virtual large aperture. The catch is that these sensors must maintain a constant spatial relationship with each other, to within a tiny fraction of a wavelength of the signal being detected.

The NASA Terrestrial Planet Finder program, for example, envisions an array of mid-infrared sensors on individual satellites, each satellite separated from its neighbors by a few hundred meters, working together to form an interferometer (Figure 1). It is a challenging project, and one of the more challenging aspects is stabilizing the formation of sensor-bearing satellites.

Figure 2. Simple and complex satellite formations might be stabilized with the laser-thruster concept. The individual satellites, separated by hundreds or thousands of meters, would be balanced by a repulsive force from the laser thrusters and an attractive force from a Kevlar tether.The concept for stabilizing a formation of satellites, as put forth by Bae, involves using laser thrust for the repulsive force between the spacecraft and a mechanical (Kevlar) tether between them as the attractive force (Figure 2). The photon force exerted by a simple laser beam — even from a moderately powerful laser — would be vastly inadequate to balance even the tiniest satellite. Instead, Bae proposes using the intracavity power of a relatively low power laser, thereby multiplying the available thrust by three or four orders of magnitude. In other words, he wants to stretch a laser resonator between two satellites that are separated by hundreds or thousands of meters (Figure 3).

Anyone who has aligned laser mirrors will look askance at this idea. Laser mirrors are hard enough to align on a granite table, so putting the two ends of a laser on two spacecraft separated by a kilometer or more seems, well, daunting. But Bae calculates that, by using a confocal resonator and allowing multiple transverse modes to oscillate in it, the angular tolerance on each mirror will be a quarter-degree. And that, he said, is easily achieved in a spacecraft by placing photodetectors behind the mirrors to provide error signals that drive piezoelectric mirror mounts.

Figure 3. The push-pull satellite-stabilization system utilizes the photon pressure exerted by a multikilowatt intracavity laser beam to provide the push (top), a Kevlar tether to provide the pull (bottom), and an unequal-arm Mach Zehnder interferometer to accurately measure the distance between satellites (center). Part of the laser light transmitted through a high-reflecting mirror illuminates the interferometer.
Maintaining the alignment of the Mach-Zehnder interferometer is more challenging, Bae concedes. Those mirrors are roughly an order of magnitude less tolerant of misalignment than the laser’s mirrors, he explained. Nonetheless, he believes that the necessary optical alignments of various components can be established and maintained in space.

Mirror contamination is another issue that might occur to dubious minds. Space dust and intense ultraviolet radiation cannot be good for laser mirrors. Bae said that “baffles” — long tubes extending out from the mirrors — could effectively limit their field of view and protect them from contamination and damage.

He also has analyzed other potential problems with the scheme, such as damping vibrational modes of the tethered satellites and correcting for damage done to the tethers as a result of collisions with tiny particles in space. His demonstration of the 35-μN thrust was conducted with funding from NASA’s Institute for Advanced Concepts.